Single-Molecule Force Spectroscopy Reveals Cation- Interactions in Aqueous Media Are Highly Affected by Cation Dehydration

Cation-pi interactions underlie many important processes in biology and materials science. However, experimental investigations of cation-pi interactions in aqueous media remain challenging. Here, we studied the cation-pi binding strength and mechanism by pulling two hydrophobic polymers with distinct cation binding properties, i.e., poly-pentafluorostyrene and polystyrene, in aqueous media using single-molecule force spectroscopy and nuclear magnetic resonance measurement. We found that the interaction strengths linearly depend on the cation concentrations, following the order of Li+ < NH4+ < Na+ < K+. The binding energies are 0.03-0.23 kJ mol(-1) M-1. This order is distinct from the strength of cation-p interactions in gas phase and may be caused by the different dehydration ability of the cations. Taken together, our method provides a unique perspective to investigate cation-pi interactions under physiologically relevant conditions.